1. Field of the Invention
The present invention relates to a temperature control apparatus for controlling a heater of a copying machine or the like.
2. Description of the Prior Art
In a copying machine or the like, in order to prevent overheating of a heater for a thermal fixer, a temperature fuse is connected in series with the heater. This fuse is arranged in the vicinity of the fixing roller of the thermal fixer. When an abnormal temperature increase near the fixing roller is detected, the fuse is disconnected.
However, this type of circuit arrangement is subject to variations in the disconnection time of the fuse depending upon the initial power-on time, the copying time and the thermal response of the fuse. Especially when a high-power heater is used for the purpose of obtaining good thermal rising characteristics, such variations in the disconnection time of the fuse are enhanced. For this reason, the fixer may be damaged (deformation of the fixing rollers, the separation pawl or the like). In the worst case, the copying machine cannot be used again.
In view of this problem, various abnormality detection measures are conventionally taken including disconnection of a temperature detection element for temperature control (e.g., a thermistor) of a heater, thereby preventing overheating of the heater. However, when overheating of the heater is caused due to short-circuiting of an element such as a triac for directly controlling turn-on/turn-off of the heater or an element constituting a triac trigger circuit, temperature control can only be performed by the temperature fuse described above.
According to another abnormality prevention method, the heater is turned off upon detecting that the heater has been turned on for a time period exceeding a predetermined ON time period. In this case, the predetermined time period over which abnormal conduction is detected is set to be a time period T1 required by the heater to reach the standby temperature (e.g., 180.degree. C.) from ambient temperature. If the heater is turned on for a time period exceeding the time period T1, an abnormality is detected. However, the time period T1 is extremely large as compared to an ON time period T2 in the standby or copying mode. For example, the time period T1 is about 20 sec., while the time period T2 is about 1 to 2 seconds. Therefore, even if an abnormality is detected in the case that the ON state of the heater for a time period exceeds the time period T1, a heater abnormality which is caused in the standby or copying mode remains undetected. Then, the heater temperature rises abnormally, and the apparatus is damaged.